A perfluoropolymer is used for coating of semiconductor production equipments or chemical plants, coating of electric cables or optical fibers, automobile components, and so on.
A perfluoropolymer immediately after obtained by polymerizing a perfluoromonomer, such as a perfluoro carbon, has unstable groups, such as —COOH groups, —CF═CF2 groups, —COF groups, —CF2H groups or —CH2OH groups, in some of its molecular chain terminals (hereinafter referred to as unstable terminal groups). It has been known that a polymer having many such terminal groups is thermally unstable, and may cause foaming or generation of hydrofluoric acid at the time of shape forming, which results in failure in forming or corrosion of a die of a forming equipment.
Then, engineering techniques for stabilizing such unstable terminal groups have been studied and some proposals have been made. For example, pellets of PFA (a copolymer of tetrafluoroethylene with a perfluoro(alkyl vinyl ether)), terminals of which are stabilized by fluorine gas treatment, have been known (Patent Document 1). Regarding such pellets, if they are intended to be made into a composition with another material, the pellets keep their shape before melting, and thus mixing with another material tends to be insufficient.
And, it has been known that a perfluoropolymer having an aliphatic ring structure in its main chain is terminal-stabilized by fluorine gas treatment (Patent Document 2). That is a process wherein, at a high temperature of at least the glass-transition point, a polymer having a fluorinated aliphatic ring structure and unstable terminal groups derived from a polymerization initiator and/or a chain transfer agent, is directly contacted with fluorine gas to convert the unstable terminal groups into stable terminal groups. However, by this process, there is a problem that particles of the polymer will melt, and the resulting polymer becomes a solid plate, which is difficult to handle.
Also, it has been known that a sheet-shaped perfluoropolymer is terminal-stabilized by fluorine gas treatment (Patent Document 3).
In processes of treating these solid perfluoropolymers with fluorine gas, there are also problems as follows:
(i) It takes a few hours to raise or lower the temperature of a pressure-resistant reactor;
(ii) When a polymer sheet is to be melted and fluorinated, if the melted polymer sheet is thick, it takes a few hours to fluorinate the polymer sheet to the core;
(iii) When pellets are used, it also takes time to diffuse fluorine gas;
(iv) When the thickness of the polymer sheet is made to be thin to shorten the time for fluorination treatment, the amount of the perfluoropolymer which can be produced by one treatment will be less;
(v) Before a perfluoropolymer is withdrawn from a pressure-resistant reactor, fluorine gas is necessarily removed from the reactor, but it takes a long time to remove the fluorine gas from the pressure-resistant reactor and replace it with nitrogen gas; and
(vi) It is a batch type process and fluorination treatment is not continuous.
Additionally, it has been known that, with regard to a soluble perfluoropolymer having an aliphatic ring structure in its main chain, a polymer dissolved in an inert solvent is fluorinated with fluorine gas (Patent Documents 4 and 5). However, with respect to this method, there are problems such that it cannot be applied to a perfluoropolymer which is not soluble to an inert solvent, the process is cumbersome and requires a high cost for treatment, wherein the polymer is necessarily dissolved in a solvent and the solvent is necessarily removed after the fluorination, and continuous processing is difficult and thus it is difficult to increase the productivity.    Patent Document 1: JP-A-62-104822    Patent Document 2: JP-A-4-189802    Patent Document 3: WO2004/102714 (Example 1 on page 9)    Patent Document 4: JP-A-11-152310    Patent Document 5: JP-A-2002-348315